The invention relates to a semiconductor device and a method of fabricating the same, and more particularly to enhancement of a fabrication yield of a semiconductor device.
In response to requirement for an electronic device to accomplish higher performance, to be smaller in size and lighter in weight, and to be able to operate at a higher rate, many new semiconductor devices have been developed. For instance, an electronic device is formed smaller in size and lighter in weight by more highly integrating a semiconductor chip to thereby fabricate a semiconductor device in smaller size and weight.
FIGS. 1A to 1C illustrate a conventional semiconductor device. FIG. 1A is a cross-sectional view of a conventional semiconductor device, FIG. 1B is an enlarged view of a connection between a wiring layer and a solder ball, and FIG. 1C is a cross-sectional view illustrating that the semiconductor device illustrated in FIG. 1A is electrically connected to a printed wiring board.
With reference to FIG. 1A, the conventional semiconductor device is comprised of a semiconductor chip 1, a film substrate 3, a polyimide adhesive layer 31 adhering the film substrate 3 to the semiconductor chip 1, a resist 32 covering the wiring layer 2 therewith and formed with a land 7 which is a recess formed at a surface thereof, a wiring layer 2 formed on the film substrate 3, a solder ball 101 mounted on the wiring layer 2 in the land 7, a gold (Au) layer 8 covering the wiring layer 2 therewith within the land 7, a metal 5 filled in a through-hole 4 formed through both the film substrate 3 and the polyimide adhesive layer 31, and a gold (Au) layer 6 covering therewith the metal 5 at a top surface thereof
The wiring layer 2 is in electrical connection with the semiconductor chip 1, and contains copper (Cu) therein. The solder ball 101 contains tin (Sn) therein. Though FIG. 1A explicitly illustrates the gold layer 8 formed on the wiring layer 2, it is considered that the gold layer 8 is diffused into the solder ball 101 in a heating step such as a temperature cycle test to be carried out after completion of the semiconductor device, and hence, it is further considered that the gold layer 8 will not exist on the wiring layer 2 after such a heating step is carried out.
As illustrated in FIG. 1C, the semiconductor device illustrated in FIG. 1A is electrically connected to a printed wiring board 34 through the solder ball 101.
FIG. 2A is an enlarged view of the wiring layer 2 and the solder ball 101, wherein the solder ball 101 is composed of Snxe2x80x94Pb eutectic solder containing tin 82 at 63% and lead 83 at 37%. FIG. 2B is a partially enlarged view of a boundary between the wiring layer 2 and the solder ball 101. As illustrated in FIG. 2B, a Cuxe2x80x94Sn alloy layer 81 having a thickness of 1.75 micrometer to 2.0 micrometer is sandwiched between the wiring layer 2 and the solder ball 101.
A method of fabricating the conventional semiconductor device illustrated in FIGS. 1A to 1C is explained hereinbelow with reference to FIGS. 3A to 3H and 4A to 4H. FIGS. 3A to 3H are partial cross-sectional views of the semiconductor device illustrated in FIG. 1A.
First, as illustrated in FIG. 3A, the wiring layer 2 containing copper therein is formed on an upper surface of the film substrate 3, and the adhesive layer 31 is formed on a lower surface of the film substrate 3. The film substrate 3 has a thickness of 12 micrometers, the wiring layer 2 has a thickness of 18 micrometers, and the adhesive layer 31 has a thickness of 10 micrometers. The film substrate 3 is composed of a material having a resistant to a temperature of 250 degrees centigrade or greater, such as polyimide. The adhesive layer 31 is composed of polyimide.
Then, as illustrated in FIG. 3B, the wiring layer 2 is patterned into a predetermined pattern.
Then, as illustrated in FIG. 3C, the patterned wiring layer 2 and the film substrate 3 are both covered with the resist 32.
Then, as illustrated in FIG. 3D, there is formed the through-hole 4 through the adhesive layer 31 and the wiring layer 2, for instance, by means of a laser beam gun.
Then, as illustrated in FIG. 3E, the through-hole 4 is filled with the metal 5 such as copper or aluminum.
Then, as illustrated in FIG. 3F, the metal 5 is covered at a top surface thereof with the gold layer 6.
Then, as illustrated in FIG. 3G, the resist 32 is formed with the land 7 above the wiring layer 2. In later steps, the solder ball 101 is mounted on the wiring layer 2 in the land 7.
Then, as illustrated in FIG. 3H, the land 7 is covered with the thin gold layer 8.
Thus, there is completed a tape substrate 93 comprised of the film substrate 3 having the wiring layer 2 formed on the upper surface. Though FIGS. 3A to 3H illustrate only one land 7, it should be noted that the film substrate 3 is in the form of a sheet, and the film substrate 3 is formed with a plurality of lands 7 in each of which the solder ball 101 is to be mounted in later steps.
A method of mounting solder balls on the wiring layer 2 is explained hereinbelow with reference to FIGS. 4A to 4H.
First, as illustrated in FIG. 4A, the tape substrate 93 resulted from the steps illustrated in FIGS. 3A to 3H is placed with the gold layer 6 being upwardly directed. For simplification, the gold layer 6 is not illustrated in FIG. 4A.
Then, as illustrated in FIG. 4B, stiffeners 41 are adhered on the tape substrate 93 at opposite its opposite ends. The stiffeners 41 are composed of copper or stainless, for instance. The stiffeners 41 are used for fixing the tape substrate 93 when the semiconductor chip 1 is mounted onto the tape substrate 93.
Then, as illustrated in FIG. 4C, the semiconductor chip 1 is mounted on the tape substrate 93, surrounded by the stiffeners 41. Then, the gold layer 6 is electrically connected to an electrode 12 of the semiconductor chip 1 through a bonding wire (not illustrated). The connection is accomplished by means of a bonding tool, a heater and a ultrasonic wave generator (not illustrated).
Then, as illustrated in FIG. 4D, resin 42 is applied between the tape substrate 93 and the semiconductor chip 1, and then, is cured to thereby reinforce a connection between the tape substrate 93 and the semiconductor chip 1. The resin 42 is composed of epoxy resin in liquid, for instance.
Then, as illustrated in FIG. 4E, a product resulted from the step illustrated in FIG. 4D is covered with a cover 43 for the purpose of protection of the semiconductor chip 1. Then, the cover 43 is sealed under atmospheric pressure. The cover 43 is made of Cu, Al or SiC, for instance.
Electrically conductive adhesive 44 such as Ag paste or Cu paste is coated on a lower surface of the cover 43. The adhesive 44 is heated, and hence, cured when the cover 43 is sealed.
Then, as illustrated in FIG. 4F, the solder balls 101 are absorbed to a positioner 45, and subsequently, the solder balls 101 are mounted onto the lands 7 formed above the wiring layer 2. Thereafter, flux (not illustrated) is applied across the solder balls 101 and the lands 7. Then, the solder balls 7 are caused to reflow to thereby physically connect the solder balls 101 to the lands 7. Then, the flux is washed out.
Thus, there is completed such a product as illustrated in FIG. 4G. The product is then subject to a temperature cycle test in order to test performances and resistance to damages.
Then, as illustrated in FIG. 4H, the solder balls 101 are caused to reflow to thereby physically connect the solder balls 101 to the printed wiring board 34. Thus, there is completed a semiconductor device as a final product.
However, the conventional semiconductor device illustrated in FIGS. 1A to 1C is accompanied with the following problem.
As mentioned earlier, a semiconductor chip is highly and highly integrated in order to meet requirement of fabricating a semiconductor device in a smaller size and a smaller weight. In a more highly integrated semiconductor chip, the greater number of pins has to be formed per a unit area of a semiconductor chip, resulting in a smaller spacing between adjacent pins. As the number of pins is increased, the number of external terminals, that is, solder balls to be connected to one semiconductor chip is also increased, resulting in a smaller spacing between adjacent solder balls.
In general, as a spacing between adjacent solder balls is decreased, a contact area through which a solder ball is connected to a wiring layer is also decreased. As a result, a connection strength between a solder ball and a wiring layer is reduced.
As explained above, in the conventional semiconductor device illustrated in FIGS. 1A to 1C, since a contact area through which the solder ball 101 makes contact with the wiring layer 2 is relatively small, a connection strength between the solder ball 101 and the wiring layer 2 is resultingly small. Accordingly, a connection at which the solder ball 101 is connected to the wiring layer 2 is likely to be broken or cracked, resulting in separation of the solder ball 101 from the wiring layer 2. This results in deterioration in fabrication yield of the semiconductor device.
In view of the above-mentioned problem, it is an object of the present invention to provide a semiconductor device which is capable of preventing separation of a solder ball from a wiring layer, and further, to provide a method of fabricating a semiconductor device which method is capable of doing the same.
In order to solve the above-mentioned problem, the inventors had studied enhancement in strength at a connection between a solder ball and a wiring layer. As a result of the long-term study, the inventors had found out that it would be possible to enhance the strength by forming a copper-tin alloy layer between a solder ball and a wiring layer to thereby prevent occurrence of breakage and cracking at a connection between a solder ball and a wiring layer. The inventors had further found out that it would be possible to form a copper-tin alloy layer by keeping a solder ball mounted on a wiring layer, at a temperature equal to or greater than a melting point of the solder ball for a predetermined period of time in inactive or reducing gas atmosphere.
Specifically, in one aspect of the present invention, there is provided a semiconductor device including (a) a semiconductor chip, (b) a wiring making electrical connection with the semiconductor chip and containing copper (Cu) therein, (c) a solder ball making contact with the wiring and containing tin (Sn) therein, and (d) a layer made of copper-tin (Cuxe2x80x94Sn) alloy, sandwiched between the wiring and the solder ball, and having a thickness equal to or greater than about 1.87 micrometers.
If the copper-tin alloy layer had a thickness smaller than 1.87 micrometers, it would be impossible to sufficiently enhance a strength at a connection between a solder ball and a wiring layer, resulting in that the connection might be broken or cracked in a heating step such as a temperature cycle test, in which case, a solder ball is separated from a wiring layer.
However, it would be possible to enhance a strength at a connection between a solder ball and a wiring layer by forming the above-mentioned copper-tin alloy layer having a thickness equal to or greater than about 1.87 micrometers, between the solder ball and the wiring layer. As a result, it would be possible to prevent or reduce occurrence of breakage or cracking at the connection, ensuring that the solder ball is not separated from the wiring layer. Thus, the present invention enhances a strength at the connection, and hence, enhances a fabrication yield of a semiconductor device.
It is preferable that the copper-tin alloy layer has a thickness equal to or greater than 2 micrometers, and more preferable that the copper-tin alloy layer has a thickness equal to or greater than 3 micrometers.
There is further provided a semiconductor device including (a) a semiconductor chip, (b) a wiring making electrical connection with the semiconductor chip, having first and second surfaces, and containing copper (Cu) therein, (c) a solder ball making contact with the wiring at the first surface and containing tin (Sn) therein, (d) a film substrate making contact with the wiring at the second surface, the film substrate being formed with a through-hole reaching the wiring, (e) an electrical conductor filling the through-hole therewith, (f) a first layer covering the electrical conductor therewith at the opposite side of the wiring, and made of gold, and (g) a second layer made of copper-tin alloy, sandwiched between the wiring and the solder ball, and having a thickness equal to or greater than about 1.87 micrometers.
The above-mentioned semiconductor chip makes it possible to enhance a strength at a connection between a solder ball and a wiring layer by forming the above-mentioned copper-tin alloy layer having a thickness equal to or greater than about 1.87 micrometers, between the solder ball and the wiring layer. As a result, it would be possible to prevent or reduce occurrence of breakage or cracking at the connection, ensuring that the solder ball is not separated from the wiring layer. Thus, the present invention enhances a strength at the connection, and hence, enhances a fabrication yield of a semiconductor device.
It is preferable that the solder ball contains agglomerates scattering therein, the agglomerates being composed of material other than tin.
For instance, the agglomerates are composed of lead (Pb) scattering at a density of 20xc3x97104 mmxe2x88x923 or greater.
If the lead agglomerates scatters at a density smaller than 20xc3x97104 mmxe2x88x923, a solder ball is likely to be deformed by the lead agglomerates, in which case, a connection between a solder ball and a wiring layer might be broken and cracked in a heating step such as a temperature cycle test, and hence, it would be impossible to prevent separation of the solder ball from the wiring layer.
However, if a solder ball contains lead agglomerates at a density of 20xc3x97104 mmxe2x88x923 or greater, it would be possible to prevent deformation of the solder ball, and hence, occurrence of breakage and cracking at the connection. As a result, it would be possible to prevent separation of the solder ball from the wiring layer which separation is caused by the above-mentioned breakage and cracking of the connection. Thus, the present invention enhances a strength at the connection, and hence, enhances a fabrication yield of a semiconductor device
It is preferable that the lead agglomerates have a cross-sectional area of 10 square micrometer or smaller in average.
If the lead agglomerates have a cross-sectional area greater than 10 square micrometer in average, a solder ball is likely to be deformed by the lead agglomerates, in which case, a connection between a solder ball and a wiring layer might be broken and cracked in a heating step such as a temperature cycle test, and hence, it would be impossible to prevent separation of the solder ball from the wiring layer.
However, if the lead agglomerates have a cross-sectional area of 10 square micrometer or smaller in average, it would be possible to prevent deformation of the solder ball, and hence, occurrence of breakage and cracking at the connection. As a result, it would be possible to prevent separation of the solder ball from the wiring layer which separation is caused by the above-mentioned breakage and cracking of the connection. Thus, the present invention enhances a strength at the connection, and hence, enhances a fabrication yield of a semiconductor device
The semiconductor device may further include a printed wiring board making electrical connection with the solder ball.
In another aspect of the present invention, there is provided a method of fabricating a semiconductor device, including the steps of (a) forming a wiring containing copper (Cu), on a substrate, (b) mounting a solder ball containing tin (Sn), on the wiring, and (c) keeping a product resulted from the step (b), in inactive gas atmosphere or in reducing gas atmosphere at a temperature equal to or greater than a melting point of the solder ball.
In accordance with the above-mentioned method, tin contained in the solder ball is diffused in liquid into the wiring layer containing copper, resulting in that a copper-tin alloy layer is formed between the solder ball and the wiring layer, having a thickness sufficient to prevent the solder ball from being separated from the wiring layer. The thus formed copper-tin alloy layer enhances a strength of a connection between the solder ball and the wiring layer, and hence, ensures that the connection is not broken and cracked. Thus, the present invention enhances a strength at the connection, and hence, enhances a fabrication yield of a semiconductor device.
The product resulted from the step (b) is kept in an inactive or reducing gas atmosphere for a predetermined period of time sufficient to enhance a strength of a connection between the solder ball and the wiring layer.
For instance, the product is kept in an inactive or reducing gas atmosphere for an hour or longer.
There is further provided a method of fabricating a semiconductor device, including the steps of (a) forming a wiring containing copper (Cu), on a substrate, (b) covering the wiring with a resist having a recess through which the wiring appears, (c) forming a through-hole through the substrate so that the through-hole reaches the wiring, (d) filling electrical conductor in the through-hole, (e) covering the electrical conductor with a gold layer, (f) connecting the substrate to a semiconductor chip. (g) mounting a solder ball containing tin (Sn), in the recess, and (h) keeping a product resulted from the step (g), in inactive gas atmosphere or in reducing gas atmosphere at a temperature equal to or greater than a melting point of the solder ball.
In accordance with the above-mentioned method, tin contained in the solder ball is diffused in liquid into the wiring layer containing copper, resulting in that a copper-tin alloy layer is formed between the solder ball and the wiring layer, having a thickness sufficient to prevent the solder ball from being separated from the wiring layer. The thus formed copper-tin alloy layer enhances a strength of a connection between the solder ball and the wiring layer, and hence, ensures that the connection is not broken and cracked. Thus, the present invention enhances a strength at the connection, and hence, enhances a fabrication yield of a semiconductor device.
The method may further include the step of forming a gold layer on the wiring in advance of mounting the solder ball on the wiring.
By forming a gold layer on the wiring and then mounting the solder ball on the gold layer, it would be possible to prevent the wiring layer from being oxidized.
There is provided a semiconductor device including (a) a semiconductor chip, (b) a wiring making electrical connection with the semiconductor chip and containing copper (Cu) therein, (c) a solder ball making contact with the wiring and containing tin (Sn) therein, and (d) a layer made of copper-tin (Cuxe2x80x94Sn) alloy, sandwiched between the wiring and the solder ball, and having a thickness equal to or greater than about 1.87 micrometers, the semiconductor device being resulted from the steps of (a) forming the wiring on a substrate, (b) mounting the solder ball on the wiring, and (c) keeping a product resulted from the step (b), in inactive gas atmosphere or in reducing gas atmosphere at a temperature equal to or greater than a melting point of the solder ball.
There is further provided a semiconductor device including (a) a semiconductor chip, (b) a wiring making electrical connection with the semiconductor chip and containing copper (Cu) therein, and (c) a solder ball making contact with the wiring and containing tin (Sn) therein, the solder ball containing lead (Pb) agglomerates scattering therein at a density of 20xc3x97104 mmxe2x88x923 or greater, the semiconductor device being resulted from the steps of (a) forming the wiring on a substrate, (b) mounting the solder ball on the wiring, and (c) keeping a product resulted from the step (b), in inactive gas atmosphere or in reducing gas atmosphere at a temperature equal to or greater than a melting point of the solder ball.
There is still further provided a semiconductor device including (a) a semiconductor chip, (b) a wiring making electrical connection with the semiconductor chip and containing copper (Cu) therein, and (c) a solder ball making contact with the wiring and containing tin (Sn) therein, the solder ball containing lead (Pb) agglomerates scattering therein, the lead agglomerates having a cross-sectional area of 10 square micrometer or smaller in average, the semiconductor device being resulted from the steps of (a) forming the wiring on a substrate, (b) mounting the solder ball on the wiring, and (c) keeping a product resulted from the step (b), in inactive gas atmosphere or in reducing gas atmosphere at a temperature equal to or greater than a melting point of the solder ball.
The above and other objects and advantageous features of the present invention will be made apparent from the following description made with reference to the accompanying drawings, in which like reference characters designate the same or similar parts throughout the drawings.